This invention lies in the field of cardiac pacing systems and methods and, more particularly, dual chamber pacing systems that detect a sudden drop in the patient""s spontaneous heart rate throughout the heart rate spectrum and provide intervention pacing when such sudden heart rate drop is detected.
It is well known that certain patients experience loss of consciousness due to a sudden drop in heart rate, which can be associated with a form of syncope. In many such patients, intervention by pacing may be beneficial for treating the symptoms. Many programmable pacemaker systems, particular dual chamber systems, have incorporated the capability of sudden rate drop (SRD) detection and responsive intervention pacing, frequently with the goal of maintaining AV synchrony as much as possible. An example of such a pacing system that provides improved sudden rate drop detection over the full range of physiological rates is found in U.S. Pat. No. 5,991,659, incorporated herein by reference in its entirety. This system, as is the case generally with other rate drop detection systems in the pacing field, relies solely on analysis of rate changes.
It is known that QT interval, being the time between contraction and repolarization, generally decreases non-linearly with heart rate and, conversely, increases with the RR heartbeat interval. Variation of the QT interval with the RR interval is generally described as having two components. A first component simply accompanies the RR interval, such that an increased RR interval results in a substantially linear lengthening of all parts of the heartbeat, including the QT interval (xe2x80x9cQTxe2x80x9d, or xe2x80x9cQT_intxe2x80x9d), and vice versa. A second component of QT variation derives from the autonomic control of the heart, leading to an overall non-linear variation of QT with heart rate, as is well documented. Generally, the contribution of autonomic modulation is greater when the heart rate is higher or increasing. For most patients, the variation of QT with rate, i.e., the QT(RR) curve, remains chronically the same. However, there are certain patients in whom changes in their QT(RR) curve occur, which changes can reliably indicate syncope or other cardiac conditions. For example, in patients with evolving neurally-mediated syncope (NMS), the normal QT vs. cycle length relation, or QT(RR), is known to change. Specifically, it has been shown that such patients exhibit a significant shortening of QTc (defined as QTRR) at rates near the rest rate, meaning that QT does not prolong in a normal fashion as heartbeat interval lengthens. As discussed in PACE, Volume 16, April 1993, Part II, Abstract #48, in patients who remained asymptomatic after head-up tilt testing, QT exhibited appropriate cycle length dependence, while QTc remained essentially unchanged. In patients who were symptomatic after head-up tilt testing, QT exhibited relatively modest prolongation, while QTc shortened significantly. See also Paradoxical Failure of QT Prolongation During Cardioinhibitory Neurocardiogenic Syncope, Am. J. Cardiology, p 100, January 1997. QT is thus a significant parameter for tracing certain variations in cardiac condition. The following patent references set forth in Table 1 suggest techniques for measuring QT for diagnostic purposes.
All patents listed in Table 1 are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the present invention.
While several approaches have been made in an effort to utilize QT for diagnostic purposes, there remains a need in the pacemaker art for a device that can efficiently and reliably detect SRD, utilizing both rate change data and QT data. Algorithms that analyze rate change data may be well tuned for some patients, but not others. There thus is a need to supplement such SRD algorithms with additional data, and to provide dual checks in order to validate SRD detection. Upon detection of SRD, the implantable cardiac device can initiate an appropriate pacing intervention.
It is an object of this invention to provide a cardiac device system and method for Sudden Rate Drop detection that is based both on continuous analysis of cardiac rate changes and continuous analysis of changes in a measure of OT. The object is to provide for sudden rate drop detection throughout the patient""s likely rate range, and to provide dual tests, or checks, to determine SRD for patients susceptible to NMS. It is another object to initiate an intervention therapy as soon as possible, and to provide for a programmable choice of what intervention therapy is to be automatically performed.
In accord with the above objects, there is provided a pacemaker system having dual algorithms for detecting the onset of SRD. A first algorithm continuously monitors changes in rate, and is exemplified by the SRD algorithm disclosed in U.S. Pat. No. 5,991,659, de Vries et al, which is incorporated herein by reference in its entirety. A second algorithm is based upon examination of changes in a measure of OT interval of such a nature that suggest the onset of NMS. In a preferred embodiment, the QT algorithm monitors differential changes in QTc, and indicates a detection of SRD when such differentials meet predetermined criteria.
The system and method of this invention further provide for a selection of one of two or more intervention therapies, depending upon whether SRD has been detected on the basis of rate changes, QT changes or both. The system provides for the physician attending the patient to program which interventions are selected and under what conditions. Thus, the physician may select a first intervention therapy in the event of SRD detection based on rate changes alone or QT changes alone, and a second intervention therapy when SRD is detected based both on rate and QT changes.